Oxide-nitride stack gate dielectric

ABSTRACT

A method of making a semiconductor structure comprises forming an oxide layer on a substrate; forming a silicon nitride layer on the oxide layer; annealing the layers in NO; and annealing the layers in ammonia. The equivalent oxide thickness of the oxide layer and the silicon nitride layer together is at most 25 Angstroms.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/506,713 filed 26 Sep. 2003.

BACKGROUND

Modern integrated circuits are constructed with up to several millionactive devices, such as transistors and capacitors, formed in and on asemiconductor substrate. Interconnections between the active devices arecreated by providing a plurality of conductive interconnection layers,such as polycrystalline silicon and metal, which are etched to formconductors for carrying signals. The conductive layers and interlayerdielectrics are deposited on the silicon substrate wafer in succession,with each layer being, for example, on the order of 1 micron inthickness.

A gate structure is an element of a transistor. FIG. 1 illustrates anexample of a gate stack 8. A semiconductor substrate 10 supports a gateinsulating layer 16, which overlaps doped regions (source/drain regions)in the substrate (12 and 14), and the gate insulating layer supports agate 18, which is typically polycrystalline silicon. On the gate is ametallic layer 30. The metallic layer may be separated from the gate byone or more other layers, such as nitrides, oxides, or silicides,illustrated collectively as barrier layer 20. The metallic layer may inturn support one or more other layers (collectively 40), such asnitrides, oxides, or silicides. Oxide 22 may be formed on the sides ofthe gate to protect the gate oxide at the foot of the gate stack; andinsulating spacers 24 may be formed on either side of the gate stack.Furthermore, contacts to the source/drain regions in the substrate, andto the gate structure, may be formed.

The continuous scaling of VLSI technologies has demanded a gatedielectric that is scaled down in thickness while maintaining therequired leakage performance. Silicon dioxide met these requirementsdown to a thickness of about 25 Angstroms. Below this thickness it firstbecomes marginal for leakage; then the thickness control itself andfinally the problem of boron penetration from the polysilicon on thegate oxide into the substrate becomes a very critical issue as thetechnologies moves to P⁺ poly gate for PMOSFETs for better performance.Nitrided SiO₂, in which nitrogen is incorporated (2 to 3%) by annealingthe gate oxide in N₂O or NO, has been proposed. This dielectric isrobust for boron penetration, due to the fact that nitrided SiO₂ isbetter for leakage because of the slightly higher dielectric constant.This dielectric can be scaled down to about 22 to 24 Angstroms (physicalthickness), below which it fails due to leakage and boron penetration.Since NO annealing also increases the oxide thickness, there is a limitto the amount of nitrogen that can be incorporated for a requiredthickness. For technology that uses a CD (critical dimension, whichcorresponds to the width of the gate) of 70 nm and smaller, the gatedielectric thickness should be in the range 14 to 16 Angstroms EOT(equivalent oxide thickness) which cannot be met by nitrided SiO₂. A newmaterial is needed to meet all the requirements.

Current technology uses nitrided SiO₂ which is formed by first growingSiO₂ by dry or wet oxidation and the oxide is typically annealed in NO,at about 850° C. to 900° C. for at least 15 minutes, to incorporatesufficient nitrogen. It is very difficult to scale the thickness belowabout 18 Angstroms, since the oxidation is too fast and annealing in NOgrows significant amounts of oxide. The dielectric is also physicallytoo thin for an EOT of 15 to 16 Angstroms. At this thickness, there istoo much tunneling current through the dielectric, resulting in highleakage. The thin dielectric also gives rise to unacceptable boronpenetration. Incorporating more nitrogen calls for an increased annealwhich would increase the oxide thickness beyond the required limit.

BRIEF SUMMARY

In a first aspect, the present invention is a method of making asemiconductor structure, comprising forming an oxide layer on asubstrate; forming a silicon nitride layer on the oxide layer; annealingthe layers in NO; and annealing the layers in ammonia. The equivalentoxide thickness of the oxide layer and the silicon nitride layertogether is at most 25 Angstroms.

In a second aspect the present invention is a method of forming a gatedielectric, comprising forming an oxide layer on a substrate; forming asilicon nitride layer on the oxide layer; and annealing the layers in NOand ammonia. The oxide layer has a thickness of 6-10 Angstroms, and thesilicon nitride layer has a thickness of 10-30 Angstroms.

In a third aspect, the present invention is a semiconductor structure,comprising a substrate, an oxide layer on the substrate, and a siliconnitride layer on the oxide layer. The oxide layer has a thickness of6-10 Angstroms, and the silicon nitride layer has a thickness of 10-30Angstroms.

In a fourth aspect, the present invention is semiconductor structure,comprising a substrate, an oxide layer on the substrate, and a siliconnitride layer on the oxide layer. The oxide layer together with thesilicon nitride layer have an equivalent oxide thickness of 12-25Angstroms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a gate stack structure.

FIGS. 2-8 illustrate a method of forming the structure of FIG. 9.

FIG. 9 shows a gate stack of the present invention.

FIG. 10 show the gate stack of FIG. 9 after further processing.

FIG. 11 shows the details of the gate stack dielectric.

FIG. 12 is a graph showing leakage.

DETAILED DESCRIPTION

The present invention makes use of the discovery that a bilayer gatedielectric, with the lower layer being silicon oxide, and the upperbeing silicon nitride, can be made which has an EOT of preferably 12-25Angstroms, including 13-15 Angstroms, 14 Angstroms, and 20-25 Angstroms.

The two-part gate dielectric is shown in FIG. 11. The gate insulatinglayer or dielectric layer 102, is composed of two parts: a silicon oxidelayer 104, and a silicon nitride layer 103. The gate insulating layer102 is on the substrate 100. The silicon oxide layer preferably has athickness of 6-10 Angstroms, including 7, 8 and 9 Angstroms. The siliconnitride layer preferably has a thickness of 10-30 Angstroms, including11, 12, 13, 14, 15, 16, 17,18, 19, 20, 21, 22, 23 and 24 Angstroms.

The oxide layer may be formed in a variety of ways, after removing anynative oxide from the substrate using hydrofluoric acid. One method isto use an RCA clean; another method is to rinse the substrate withdeionized water containing ozone (O₃), to form an oxide layer having athickness of about 8 Angstroms; this may be done as the rinse followingthe removal of the native oxide. In another method, low pressure(100-200 mTorr) oxygen, optionally diluted with nitrogen or anotherinert gas, is used to thermally grow the oxide layer. In still anothermethod, the oxide layer may be formed by steam oxidation.

Once the oxide is formed, the silicon nitride layer may be formed. Thismay be done by forming nitrogen rich silicon nitride by LPCVD, usingdichlorosilane and ammonia, preferably in a ratio of 1:1000 to 1:3, morepreferably 1:100 to 1:33, most preferably 1:33. Alternatively, thesilicon nitride layer may be formed from dichlorosilane and ammonia byAtomic Layer Deposition. Once formed, the layer may be annealed in NO at800-900° C. for 15-30 minutes; and annealed in ammonia by rapid thermalannealing (RTA) for 30 seconds to 1 minute, or in a furnace for 5-10minutes. Optionally, nitrogen may be used to dilute the NO and ammoniain either annealing process.

The remainder of the semiconductor structure may be formed as follows.

Referring to FIG. 2, the gate insulating layer 102 is on thesemiconductor substrate 100. The semiconductor substrate may be aconventionally known semiconductor material. Examples of semiconductormaterials include silicon, gallium arsenide, germanium, gallium nitride,aluminum phosphide, and alloys such as Si_(1-x)Ge_(x) andAl_(x)Ga_(1-x)As, where 0≦x≦1. Preferably, the semiconductor substrateis silicon, which may be doped or undoped.

Referring to FIG. 3, a gate layer 105 may be formed on the gateinsulating layer. The gate layer may contain a variety of semiconductingmaterials. Typically, a gate layer contains polycrystalline silicon(poly) or amorphous silicon. The gate layer may be doped with one typeof dopant (P⁺ or N⁺), or it may contain both types of dopants indiscrete regions. A split gate is a gate layer containing both P⁺ and N⁺doping regions.

In the case of a split gate, those regions of the gate that are P⁺ doped(such as with B or BF₂ ⁺) are over N⁻ doped channel regions of thesubstrate, forming a PMOS device; those regions of the gate that are N⁺doped (such as with As⁺ or phosphorus⁺) are over P⁻ doped channelregions of the substrate, forming an NMOS device. The P⁺ and N⁺ dopingregions of the gate are separated by a region which is on an isolationregion of the substrate. The doping of the regions of the gate ispreferably carried out after forming the gate, by masking and dopingeach region separately, or by an overall doping of the gate with onedopant type, and then masking and doping only one region with the otherdopant type (counter doping).

Referring to FIG. 4, a barrier layer 115 may optionally be formed on thegate layer. The optional barrier layer may contain a variety ofmaterials, including nitrides, silicides, and oxides, and is preferablya conductive material. For example, the barrier layer may containrefractory silicides and nitrides. Preferably, the barrier layercontains tungsten nitride or silicide.

Referring still to FIG. 4, a metallic layer 125 may be formed on thegate layer, or the barrier layer 115, if it is present. Preferably, themetallic layer has a thickness of 200-600 angstroms, more preferably300-500 angstroms, most preferably 325-450 angstroms. The metallic layer125 may contain a variety of metal-containing materials. For example, ametallic layer may contain aluminum, copper, tantalum, titanium,tungsten, or alloys or compounds thereof. Preferably, the metallic layercomprises tungsten or titanium. The metallic layer may be formed, forexample, by physical vapor deposition (PVD) of the metal, or by lowpressure chemical vapor deposition (LPCVD) of a mixture of a metalhalide and hydrogen.

Referring to FIG. 5, an etch-stop layer 145 may be formed on themetallic layer by a variety of methods, including chemical vapordeposition (CVD). Preferably, the etch-stop layer is a nitride layer.More preferably, the etch-stop layer is silicon nitride formed by PECVD.The etch-stop layer may vary in composition, so that the top of theetch-stop layer is anti-reflective, for example so that the top of theetch-stop layer is silicon rich silicon nitride, or silicon oxynitride;this layer may also act as a hard mask to protect the etch-stop layerduring subsequent etches. Alternatively, a separate anti-reflectivelayer (ARC) may be formed.

Preferably, the etch-stop layer is formed rapidly at a relatively lowtemperature. For example, if the gate layer contains both P⁺ and N⁺doping regions, diffusion of the dopants may occur if the wafer ismaintained at sufficiently high temperatures for a prolonged period oftime. Thus, it is desirable that any high temperature processing isperformed only for relatively short periods of time. Likewise, it isdesirable that any lengthy processing is carried out at relatively lowtemperatures. Preferably, the etch-stop layer is formed at a temperatureof at most 750° C., if the atmosphere is substantially devoid of oxygen,or in a reducing environment (hydrogen rich). Under typical conditions,a temperature of at most 600° C. is preferred, at most 450° C. is morepreferred. A temperature of at least 350° C. is preferred, such as 400°C. The depositing of the etch-stop layer is preferably carried out at atemperature and for a time that does not result in substantial diffusionbetween the P⁺ region and the N⁺ region in a split gate.

Referring to FIGS. 6-9, each layer may be patterned to form the gatestack. The patterning may be accomplished, for example, by conventionalphotolithographic and etching techniques. Referring to FIGS. 6 and 7,the etch-stop layer may be etched to form a patterned etch-stop layer150, for example by forming a patterned photoresist 210 on etch-stoplayer 145 (FIG. 6) and then etching the exposed portions of the layer. Ahydrofluoric acid dip may be used to remove sidewall passivation.

The etch-stop etching may be carried out by exposure to a plasma formedfrom a mixture of gasses. Preferably, the gasses and plasma comprisecarbon, fluorine and hydrogen. Preferably, the atomic ratio offluorine:hydrogen is 43:1 to 13:3, more preferably 35:1 to 5:1, mostpreferably 27:1 to 7:1. Preferably, the mixture of gasses includes CF₄and CHF₃; preferably the ratio by volume of CF₄: CHF₃ is 10:1 to 1:3,more preferably 8:1 to 1:2, most preferably 6:1 to 1:1. The gas mixtureand plasma may also include other gasses, such as He, Ne or Ar. Thepressure during etching is greater than 4 mTorr, preferably at least 10mTorr, such as 10-80 mTorr, more preferably at least 15 mTorr, such as15-45 mTorr, most preferably 25-35 mTorr.

The patterned etch-stop layer may be used as a hard mask for the etchingof the metallic layer 125 (FIG. 7) to form a patterned metallic layer130 (FIG. 8). The patterned etch-stop layer and the patterned metalliclayer may be used as a hard mask for the etching of the gate layer 105(FIG. 8) to form patterned gate layer 110 (FIG. 9). The gate etching maybe carried out by conventional gate etch techniques, for example byexposure to a plasma formed from chlorine, hydrobromic acid and/oroxygen.

The patterned photoresist 210 (FIG. 6) may be removed at any stage ofthe gate stack formation following the etch-stop etch. For example, thepatterned photoresist may be removed immediately after the etch-stopetch (as illustrated in FIGS. 6 and 7), or it may be removed after theetching of the metallic layer or after the gate etching. The removal ofthe photoresist may be followed by a cleaning procedure to ensureremoval of any residual byproduct of the photoresist or of thephotoresist removal. For example, the photoresist may be removed byashing the patterned photoresist to provide a gate stack containing apatterned etch-stop layer (FIG. 7). This gate stack without aphotoresist layer may then be treated with a cleaning solution tocomplete the removal and cleaning process. The most preferred cleaningagent contains water, 2-(2 aminoethoxy) ethanol, hydroxylamine, andcatechol. An example of a cleaning solution is EKC265™ (EKC, Hayward,Calif.).

FIG. 9 thus illustrates a gate stack 200 which may be formed on asemiconductor wafer. Semiconductor substrate 100 supports a gateinsulating layer 102, which in turn supports a gate layer 110. The gatelayer supports a metallic layer 130, which may optionally be separatedfrom the gate layer by barrier layer 120. The etch-stop layer 150 is onthe metallic layer 130.

Further processing of the gate structure may include forming sidewalloxide regions 170 on gate layer 110 and forming spacers 160 (preferablycontaining nitride) on the sides of the stack. Furthermore, a dielectriclayer 180 maybe formed on the etch-stop layer, and contacts or via 190formed through the dielectric to the substrate, as illustrated in FIG.10. This via may be lined and filled to form a via-contact, for examplewith TiN and tungsten, respectively. Other processing may includeforming contacts to the gate itself.

Other processing may be used to complete formation of semiconductordevices from the semiconductor structure. For example, source/drainregions 12, 14 may be formed in the substrate, additional dielectriclayers may be formed on the substrate, and contacts and metallizationlayers may be formed on these structures. These additional elements maybe formed before, during, or after formation of the gate stack.

The related processing steps, including the etching of the gate stacklayers and other steps such as polishing, cleaning, and depositionsteps, for use in the present invention are well known to those ofordinary skill in the art, and are also described in Encyclopedia ofChemical Technology, Kirk-Othmer, Volume 14, pp. 677-709 (1995);Semiconductor Device Fundamentals, Robert F. Pierret, Addison-Wesley,1996; Wolf, Silicon Processing for the VLSI Era, Lattice Press, 1986,1990, 1995 (vols 1-3, respectively), and Microchip Fabrication 4rd.edition, Peter Van Zant, McGraw-Hill, 2000.

The semiconductor structures of the present invention may beincorporated into a semiconductor device such as an integrated circuit,for example a memory cell such as an SRAM, a DRAM, an EPROM, an EEPROMetc.; a programmable logic device; a data communications device; a clockgeneration device; etc. Furthermore, any of these semiconductor devicesmay be incorporated in an electronic device, for example a computer, anairplane or an automobile.

Using the two-part gate dielectric of the invention shows a Vt(threshold voltage) for a PMOS FET slightly higher than that of SiON(−0.54 vs. −0.46), indicating no boron penetration. FIG. 12 is a graphshowing leakage on the vertical axis (A/cm²), and EOT (in Angstroms) onthe horizontal axis. In this graph, “THERMAL-NO” represent a devicehaving a thermally grown oxide, followed by annealing in NO; “VTR-NO” isoxide grown thermally in a vertical furnace, followed by annealing inNO; and O/N stack is the two-part gate dielectric of the presentinvention.

1. A method of making a semiconductor structure, comprising: forming anoxide layer, on a substrate, wherein the oxide layer contains less than2% nitrogen; forming a silicon nitride layer on the oxide layer;annealing the layers in NO; and annealing the layers in ammonia; whereinthe equivalent oxide thickness of the oxide layer and the siliconnitride layer together is at most 25 Angstroms.
 2. The method of claim1, wherein the oxide layer has a thickness of 6-10 Angstroms.
 3. Themethod of claim 1, wherein the silicon nitride layer has a thickness of10-30 Angstroms.
 4. The method of claim 1, wherein the forming of theoxide layer comprises rinsing the substrate with deionized watercontaining ozone.
 5. The method of claim 1, wherein the forming of theoxide layer comprises steam oxidation.
 6. The method of claim 1, whereinthe forming of the oxide layer comprises thermally growing the oxidelayer with oxygen.
 7. The method of claim 1, wherein the forming of thesilicon nitride layer comprises depositing the silicon nitride by LPCVD.8. The method of claim 1, wherein the annealing in NO is carried out ata temperature of 800-900° C. for 15-30 minutes.
 9. The method of claim1, wherein the annealing in ammonia is carried out by rapid thermalannealing at a temperature of 900° C. for 0.5-1 minute.
 10. The methodof claim 1, wherein the annealing in ammonia is carried out in a furnaceat a temperature of 900° C. for 5-10 minutes.
 11. The method of claim 1,wherein the semiconductor structure is a gate dielectric.
 12. A methodof forming a gate dielectric, comprising: forming an oxide layer, on asubstrate, wherein the oxide layer contains less than 2% nitrogen;forming a silicon nitride layer on the oxide layer; and annealing thelayers in NO and ammonia; wherein the oxide layer has a thickness of6-10 Angstroms, and the silicon nitride layer has a thickness of 10-30Angstroms.